A cellular radiotelephone network generally comprises a plurality of mobile stations in radio communication with a plurality of base-sites, coupled to a base-site controller, and a plurality of base site controllers coupled to a mobile switching center. There may be a plurality of mobile switching centers. The coupling means is generally via bi-directional microwave or wireline terrestrial-based circuits such as T1 ISDN links. Many channels of individual subscriber information are generally communicated over each of these links.
While the links generally employ a serial digital physical connection, logical circuits are created to carry individual channels of subscriber information. There are two traditional techniques utilized to create the logical medium partitioning--circuit and packet switching.
Circuit switching is the traditional technique utilized. In a circuit switching arrangement, each logical circuit is allocated a constant portion of the total bandwidth of the circuit. The multiplexing technique is typically time division multiple access (TDMA). The serial digital stream is multiplexed to allocate each circuit a certain number of bits per second in a predetermined way. Control bits are inserted to facilitate the proper transport of the serial physical stream.
A typical T1 circuit provides approximately 1.544 megabit/s of serial binary information. A typical logical partitioning is into 24 individual PCM circuits of 64 kilobits per second each. Physically, eight bits from each circuit is, in turn, sent sequentially. Alternatively, the T1 circuit may be logically partitioned into 48 individual ADPCM circuits of 32 kilobit/s each. Physically, four bits from each ADPCM circuit is, in turn, sent sequentially.
An alternate technique to circuit switching is packet switching. In packet switching, information from each of the individual sources is buffered by a packet controller. In a predetermined way, information is transmitted in blocks using the full bandwidth of the circuit. Information blocks are encoded along with certain control and addressing information, allowing a packet controller receiving the packet the capability to properly reconstruct the individual logical channel's information.
The above techniques are traditionally utilized in digital cellular radiotelephone communication networks. Such networks have air interfaces which multiplex information from several users onto a single radio frequency (RF) resource (i.e., a channel). For example, in a time division multiplexed (TDM) system, information from several users share a common RF channel, but are transmitted in bursts, where information from each user is sequenced in time. An alternative implementation utilizes direct sequence code division multiple access (DS CDMA). In this case, all users share at the same time an RF channel and each user's data stream is distinguished by encoding it with a unique code. Use of this code at the receiver allows discrimination of one user from another.
All the above networks are slotted or framed. In such networks, information from a user is sent in blocks. For example, speech is coded in sampled blocks, generally consisting of about 20 msec of information. This information is coded into a block of information that is transmitted over an air interface. At the receiver, the blocks are decoded to create an adequate representation of the original analog sample waveform.
Delay is a common problem for all varieties of networks. It is desirable to minimize the introduced time delays since excess delay reduces perceived quality and burdens the ability of echo cancellation circuits to eliminate the effects. Therefore, cellular radiotelephone communication networks incorporating circuit switching incorporate provisions to adjust the timing of an interface means so that information is received just at the moment it is needed for transmission over the air interface. Traditional packet switching of individual circuits is not amenable to traditional delay control and minimization procedures.
A problem with circuit switching is that it generally takes 20 ms to send the sampled block of speech. That is, each 20 ms block of information is bit-by-bit time-division multiplexed onto the individual PCM circuit or broken into small sub blocks which in turn are time-division multiplexed onto the individual PCM circuit. The receive end must buffer the received bits until the entire 20 ms block is received and only then may further processing commence.
A problem arises in organizing the switching resource when the bandwidth of an individual logical circuit is not fixed. Such is the case, for example with the DS CDMA air interface specification recently standardized by the Telecommunications Industry Association (TIA) as document IS 95. This interface utilizes a variable rate speech vocoding system, standardized as TIA document IS 96, which encodes 20 msec blocks of speech at rates of 8, 4, 2 or 1 kilobit/s. The speech is determined to minimize the average rate but ensure that adequate speech quality is provided. Furthermore, timing of each logical circuit's transmitted blocks may be staggered with respect to each other. In this DS CDMA implementation, a circuit switching approach allows for trunking efficiencies when communicating information for individual users. A packet switching approach must also allow for trunking efficiencies when communicating information for individual users.
Thus a need exists for a cellular radiotelephone network having the benefits of both circuit and packet switched interface techniques while mitigating time delay introduced by these interface techniques.